Description

Geographically isolated populations of endangered species such as the Mount Graham red squirrel (MGRS) (Tamiasciurus hudsonicus grahamensis) are particularly sensitive to fluctuations in habitat quality and extent. In recent decades a series of insect outbreaks and high-severity fires degraded the sole MGRS habitat in the Pinaleño Mountains of southeast Arizona, and have raised concerns about the long-term viability of the species. In this study we use dendrochological and spatial analysis methods to reconstruct the history of MGRS habitat size and quality over the past three centuries to provide historical context for current habitat conditions. We found that EuroAmerican land uses starting in the late 1800s excluded fire from the mixed-conifer forest, resulting in a significant expansion of suitable MGRS habitat over the next century. Suitable habitat at the start of the MGRS monitoring program in 1989 was greater than at any time in the reconstructed period. Prior to EuroAmerican settlement, habitat would have been limited primarily to the spruce-fir forest by the frequency and severity of fires burning in the mixed-conifer forest at the ecotone between spruce-fir and mixed-conifer forest types. The spruce-fir and mixed-conifer forests burned in a large, high-severity fire in 1685, resulting in a much greater reduction in suitable habitat for MGRS, lasting several decades, than has been experienced by the contemporary squirrel population. Recovery of the MGRS population following the 1685 fire suggests that MGRS is highly resilient to significant habitat degradation. Recent disturbances had the greatest effects on spruce-fir forest and forecasts of future temperature and precipitation conditions suggest a decline in suitable conditions for this forest type. Over the past century MGRS populations have adapted to mixed-conifer forest, which now hosts the majority of active middens and nests. If historically-associated habitat in the spruce-fir forest continues to decline, the mixed-conifer forest may provide suitable resources for MGRS if exclusion of fire from these sites continues.